The adaptive immune system is responsible for recognizing and eliminating foreign antigens with specificity and efficiency. Defects in this system can result in a broad range of phenotypes, with decreased function leading to immunodeficiency syndromes and increased or unregulated function leading to autoimmune disorders or cancers. Therefore, it is imperative to understand the mechanisms by which cellular processes in the adaptive immune system function and how these mechanisms are precisely regulated to prevent disease. An example of this regulation is found in the activation response of T cells, a key component of the adaptive immune system. T cell activation occurs upon antigen recognition and results in rapid cellular proliferation; equally as important, however, is the requirement for the T cell to attenuate this activation response. One mechanism that T cells use to diminish their activation is by regulating gene expression through the use of pre-mRNA alternative splicing. An example of this is regulation of the gene CD45, which undergoes a signal-induced change in splicing that is crucial to attenuating T cell activation. In this proposal, I plan to further our understanding of the regulation of T cell activation-induced alternative splicing through the following specific aims: 1) Identify the trans-factors involved in the alternative splicing of CD45 using a genetic screening methods which take advantage of a dual-reporter system that amplifies changes in splicing transcriptionally; 2) Characterize the regulation of CD45 alternative splicing by the factors identified in Specific Aim 1 and other candidate factors using a combination of biochemical and cell biological methods; 3) Determine if the trans-factors regulate splicing of other genes that undergo T cell activation-induced alternative splicing using similar methods as in Specific Aim 2. Together, these studies will increase our understanding of the regulation of CD45 alternative splicing and provide insight into how the divergent signaling pathways that are initiated upon antigen recognition converge to regulate signal-responsive splicing of distinct genes during T cell activation. The adaptive immune response serves a primary role in protecting humans from disease, however, deregulation of this system can lead to a variety of disorders including immunodeficiency diseases, autoimmunity syndromes, and cancers. In this proposal, I aim to further our knowledge of the regulation of an important yet poorly characterized process that is crucial to the adaptive immune response. An increased understanding of the precise regulation required for adaptive immune system function may lead to the development of therapies for diseases in which this regulation is lost. [unreadable] [unreadable] [unreadable]